In recent years, larger capacity in an optical communication system is being developed in response to rapid increase in data communication amount. As a key device in the optical communication system, a semiconductor laser is widely used, which performs intensity modulation or phase modulation in accordance with a transmission range of an optical signal. In a short-to-intermediate range optical transmission system of 100 km or less such as a metropolitan area network (communication within a city) or a fiber to the home (FTTH) network, a small-sized device has been demanded, and an intensity modulation laser has been widely used. Meanwhile, in a long range optical transmission system of 100 km or more such as a core network (communication between cities), a phase modulation laser that realizes both of high-speed operation and long range transmission has already been in use.
As the phase modulation laser, a semiconductor Mach-Zehnder optical modulator (Mach-Zehnder Modulator, hereinafter referred to as “MZM”) is widely used. The phase modulation MZM is a device that converts an electric digital signal into an optical digital signal. In the phase modulation MZM, phase modulation is performed on an output of continuous light (CW light) from a semiconductor laser by changing a refractive index of a semiconductor quantum well (Multi-Quantum Well, hereinafter referred to as “MQW”) structure forming two optical waveguides, with the use of an electric signal.
In order to obtain satisfactory optical characteristics in the MZM, an optimal drive voltage needs to be input into the two optical waveguides. The drive voltage consists of an input electric digital signal (high frequency signal) and a direct current voltage that determines an operating point of the drive voltage. Therefore, a direct current voltage that determines the operating point needs to be applied to the two optical waveguides from an outside voltage supply, independently of the high frequency signal.
In order to input a high frequency signal and a direct current voltage into an optical waveguide, an MZM using a bias tee consisting of an inductor and a capacitor is widely used.
However, the size of the inductor incorporated into the bias tee is large, making it difficult to downsize the MZM. Hence, there was a problem in being unable to satisfy the demand of the market.
In view of this, with the purpose of downsizing the MZM, for example, Patent Document 1 describes a technology in which bias resistors having resistance values of from hundreds of ohms to several kiloohms are disposed in signal electrodes at terminating resistor parts that are respectively provided on two optical waveguides, and a direct current voltage is applied to the two optical waveguides via the bias resistors.